68kMLA Classic Interface

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Classic Clock Not Advancing
Posted by: Phipli on 2023-11-30 01:59:02
I think I know what is wrong, but hope this helps anyone else that sees similar behaviour.

My dad's Mac classic clock doesn't advance when there is no battery installed, but does when there is a battery. Looking at the schematic, this probably means that the diode D2 is bad, or the traces through it. D2 looks like it is meant to prevent the battery back-feeding into the 5V rail (D1 stops the 5V trying to charge the clock battery). With it not working, the RTC is only ever powered from the battery. This isn't a major issue, but I'll look into fixing it.

In my case I suspect a bad connection because the diode itself tested good.

Here is the pertinent area of the Bromarc schematic. With a correction where they didn't know what VIA stood for. Ignore the red dashes, that's just me marking off traces I'd checked.

1000014389.jpg
Posted by: Daniël on 2023-11-30 02:34:25
Do you have an oscilloscope? I'd check if the 32KHz clock crystal is working, given that's what the clock is running off AFAIK. If the Mac does save its PRAM settings on power off, that'd probably clear UA2/343S0042 as a potential culprit (though who knows if that chip could partially fail :-D).
Posted by: robin-fo on 2023-11-30 02:52:04
Did you check the voltages (e.g. at pin 20 of the RTC) when the board is running without battery to confirm your hypothesis ?
You also need to check the trace/via from D2 to 5V..
Posted by: LaPorta on 2023-11-30 03:00:18
You are on the right track. I had a very similar issue a few years back while working on a Classic. See if this thread helps you in any way:

Classic: Clock won't run

So I just re-caped a Classic that I received from someone. Now, I have no idea if the clock worked before (didn't check), but currently something bizarre I have never seen before is going on: The clock won't advance time. I set the clock to the correct time last night, and it just stays where it...
68kmla.org 68kmla.org
Posted by: Phipli on 2023-11-30 03:38:56
Do you have an oscilloscope? I'd check if the 32KHz clock crystal is working, given that's what the clock is running off AFAIK. If the Mac does save its PRAM settings on power off, that'd probably clear UA2/343S0042 as a potential culprit (though who knows if that chip could partially fail :-D).
The clock works when the battery is installed, so it isn't the crystal or the RTC itself.

I'm not worried, I have a likely issue and will fix it next time the board is out. The post was to notify other people with similar issues what the culprit might be.
Posted by: Phipli on 2023-11-30 03:40:22
Did you check the voltages (e.g. at pin 20 of the RTC) when the board is running without battery to confirm your hypothesis ?
You also need to check the trace/via from D2 to 5V..
Not yet, I'm more likely to just test it with a continuity tester and repair it. The fault should be easy to find if it is what I think it is.
Posted by: Phipli on 2023-11-30 03:47:39
You are on the right track. I had a very similar issue a few years back while working on a Classic. See if this thread helps you in any way:

Classic: Clock won't run

So I just re-caped a Classic that I received from someone. Now, I have no idea if the clock worked before (didn't check), but currently something bizarre I have never seen before is going on: The clock won't advance time. I set the clock to the correct time last night, and it just stays where it...
68kmla.org 68kmla.org
Your issue sounds like it was pretty similar 🙂

It didn't flag your thread when I checked the similar suggestions after writing my title, shame, I'd have posted there.
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